Quarks are the smallest particles known to man. They could be considered the ‘DNA’ of the atom, which in itself is invisible to the human eye.
Quarks are the building blocks of the proton and neutron, generically called hadrons, which form the atom.
There are six types of quarks, called flavors: Up, Down, Strange, Charm, Bottom and Top. Up and down quarks have the lowest masses. The more heavier ones change into up and down quarks via a process of particle decay, which is the transformation from a higher mass state to a lower mass state. As a result, up and down quarks are generally stable and are the most common in the universe, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions.
No one can see a quark, even with the most powerful microscope; hence, most of the scientific conclusions are made through extensive research and observations.
(For an illustration at how small these entities are, visit The Scale of the Universe and keep cruising down through the world of the micro universe, until you reach quarks, 10-18 meters in size).